1. Field of the Invention
This invention relates to casting of metals, especially casting of light metals and their alloys, and more particularly relates to increasing strength of specific portions of a casting.
2. Description of the Prior Art
In casting of metals, for example in casting of an aluminum car wheel, ejector pins are used to separate the casting from the mold. The pins conventionally are made of tool steel or hot mold steel. And in conventional casting methods, the whole mold cavity is filled up with the molten metal alone, and also no specific design is given to the surface configuration of the casting.
Since the tool steel or hot mold steel used for the conventional ejector pins has a relatively low thermal conductivity of 30.5 W/m.K (at 20.degree. C. ), those portions of the casting which contact with the pins cool down relatively slowly, so that they are not so mechanically strong as they could be. Also in the conventional casting methods, since no specific contrivance is given in filling the cavity with the molten metal and no specific design is given to the surface configuration of the casting, i.e., to the shape of the cavity, it is not possible to obtain a casting having specific portions increased in strength.
However, in case of an aluminum car wheel, for example, it is often the case that as long as certain portions of the wheel are satisfactorily strong, the strength of the rest of the wheel is not so important. It is also the case that as long as the front and decorative side of the wheel being visible when fitted to a car is smooth, the appearance of the rear of the wheel is not so important. The conventional ejector pins and casting methods mentioned above provide no means for increasing strength of specific portions of the casting, such as the portions not visible in use, and hence provide no means for decreasing the weight of the casting.